Subject: bug#26106: Defining a method named '-' with one parameter

From: Andy Wingo <[email-masked]>
Date: Wed, 19 Apr 2017 17:00:25 +0200

On Wed 15 Mar 2017 14:35, Alejandro Sanchez <[email-masked]> writes:

> If I define a method named ‘-‘ which only takes in one parameter, the expression ‘(- v)’ gets rewritten to ‘(- 0 v)’. Here is a minimal example:
>
> 	(use-modules (oop goops))
> 	
> 	(define-class <vector2> ()
> 	  (x #:init-value 0 #:getter get-x #:init-keyword #:x)
> 	  (y #:init-value 0 #:getter get-y #:init-keyword #:y))
> 	
> 	(define-method (* (n <number>) (v <vector2>))
> 	  (make <vector2> #:x (* n (get-x v)) #:y (* n (get-y v))))
> 	
> 	(define-method (- (v <vector2>))
> 	  (* -1 v))
> 	
> 	(define v (make <vector2> #:x 1 #:y 2))
> 	(* -1 v)  ; Works fine
> 	(- v)  ; Throws error
>
> Here is the error message:
>
> 	scheme@(guile-user)> (- v)
> 	ERROR: In procedure scm-error:
> 	ERROR: No applicable method for #<<generic> - (2)> in call (- 0 #<<vector2> 10a8e4020>)
> 	
> 	Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
> 	scheme@(guile-user) [1]> ,bt
> 	In current input:
> 	     23:0  2 (_)
> 	In oop/goops.scm:
> 	   1438:4  1 (cache-miss 0 #<<vector2> 10a8e4020>)
> 	In unknown file:
> 	           0 (scm-error goops-error #f "No applicable method for ~S in call ~S" (#<<generic> - (2)> (- 0 #<<vec…>)) #)

Is (- x) equivalent to (* x -1) ?

Right now there are a few things happening.  The "primitive expansion"
phase in an early part of the compiler turns (- x) to (- 0 x), where
obviously it should not be doing that.  But can it turn it into (* x -1)
?  Note that somewhat confusingly, a later part of the compiler that can
detect when X is a real number will undo that transformation, turning it
to (- 0 x) when X is real.  So that sounds OK from an optimization point
of view but is the (* x -1) tranformation correct from the math point of
view?

Andy